Historically, the operation has been performed in the case of wire or mesh by withdrawing the wire or mesh vertically from the molten metal bath through a bed of particulate material with the coating control mechanism acting at the point of emergence of the object. Traditionally, the oiled charcoal process has been used in this way for the production of heavy coatings and more recently, a Gas Wiping Process of the type disclosed in Australian Pat. No. 421,751 has been employed in the galvanizing of steel wire. To achieve thin coatings, tighter wiping techniques are employed such as those utilising asbestos pads pressed hard around the moving wire at its exit from the coating bath.
In the case of strip, two methods have been used to control coatings. The first technique relies on coating rolls rotating in close proximity on the strip near the point of emergence from the coating bath. More recently, jet wiping techniques have been applied to the control of coatings of zinc and zinc/aluminium alloys.
With the exception of jet wiping techniques, these methods suffer from the disadvantage that no rapid and direct coating control mechanism is available.
Although with the known techniques coating weight adjustment is available, it involves other process parameters. For example, in the oiled charcoal wiping process for wire, coating weight on a 2.00 mm wire can be reduced from an average of 300 g/m.sup.2 to an average of 250 g/m.sup.2 by reducing throughput speed from 20 m/min. to 15 m/min. This raises problems not only of reduced output but with other in-line processes and adjacent products.
In the case of the jet wiping process for strip, which relies on the use of gas streams under pressure to displace molten metal from the surface of the sheet, the large volume of residual gases and the noise inherent with such a process are seen to be serious disadvantages.
In considering the withdrawal of an object from a molten metal bath, various forces act on the entrained liquid film. Of these forces, the significant force in limiting the thickness of the entrained film is the force due to gravity. In the various known commercially applied processes for wire and strip and the like, this force is assisted by additional forces, e.g., pressure from asbestos pads when used, the gas pressure barrier produced when jet wiping is used, and the limiting of gap when coating rolls are used.
As stated earlier, all these additional assisting forces (with the exception of the gas pressure barrier) do not lend themselves to continuous control over a wide range while being capable of small, precise incremental adjustments.
It is an object of this invention to seek to overcome these disabilities by providing a technique which permits the application of an additional assisting force which is capable of precise direct adjustment and monitoring. Through the application of this technique, it is possible to produce a wide range of coating weights approaching the thin coatings as produced by the tight wiping processes up to the heaviest coatings capable of being produced by hot dipped metal coating techniques.
The invention basically involves the application of an electro-magnetic force, by utilising an electromagnetic device (E.M.D.) at and below the point of emergence of the coated wire, strip or the like from the molten metal bath, such that there is inter-action between the device and the system involving both the object and the molten metal during entrainment.
It is recognised that other attempts have been made to utilise electromagnetic forces to alter the molten coating thickness on a moving object. Three prior publications identified are British Pat. No. 1,221,905 granted to Allmanna Svenaka Elektriska Aktienbolaget (A.S.E.A.) German patent application (OLS) No. 2,202,764 in the name of Demag A. G. and Japanese patent application No. 69/48599 in the name of Mitsubishi Electric Corporation. In the case of both the A.S.E.A. and Demag publications, the alteration is achieved by imposing a travelling electromagnetic field on the coating after it has formed. In the case of the Mitsubishi application, they also sought to alter the coating by the application of a travelling electromagnetic field both at the point of emergence of the object from the bath, and above.
The present invention differs from the prior art in that it seeks to effect control through the application of a stationary single coil device which is powered by an A.C. current to produce an electromagnetic field that acts on the entrained layer moving in the molten metal bath with the object to be coated. This entrained layer is the precursor of the final metal coating. This offers particular advantages in the use of the technique by way of construction of apparatus, compactness of apparatus and simplicity of power generation over previous disclosures.